Typical chest X-ray (CXR) data is a two-dimensional (2D) projection image; each pixel represents a volumetric integration. Due to human anatomy there are many lung structures which can be falsely identified as nodules in projection data. One such structure is a rib-cross, which appears when the anterior and posterior ribs overlap each other. These rib-cross areas are often falsely identified as nodules and pose problems for Computer Aided Diagnosis (CAD) systems.
The lungs are situated within the rib cage, which is basically a bone structure. The bones absorb X-rays to a greater extent than do soft tissues, and hence these appear as white stripes across the X-ray radiograph. The ribs curl around the body and consequently the rib shadow on the X-ray radiograph appear to cross each other. The cross-over points appear as a pattern. This pattern may disguise the shadows cast by nodules which may be aligned with the crossover points.
For effective diagnostics, the regions in the X-ray image which are identified as possible nodules should be verified to ascertain whether they are actually nodules or false positives. The occurrence of false positives, where some other feature such as a rib crossing shadow is mistakenly identified as being attributed to a nodule, should be minimized. When scanning the lung region of posterior/anterior X-ray images to look for regions of varying contrast or edges, a large number of candidate regions turn out to be rib crossings and the like. Hence there should be a mechanism to cross-check or verify the identified nodules.